函数隐式声明 implicit declaration of function

默认函数声明Implicit declaration function引发的血案

文件1中定义了函数，并malloc了一段内存，返回指针

char *func1(char *arg){
    char *c = (char*)malloc(size);
    return c;
}

在文件2中直接调用该函数，前提是没有显示的声明func1

    ...
    new_addr = func1("foo");
    print(*new_addr);// coredump
    ...

在func1中分配的内存地址是0x7fd370c11008，指针new_addr为0x70c11008。

C99文档中有说明：

If an attempt is made to modify the result of a function call or to access it after the next sequence point, the behavior is undefined. If the expression that denotes the called function has a type that does not include a prototype, the integer promotions are performed on each argument, and arguments that have type float are promoted to double. These are called the default argument promotions.

在64位机器上，指针的长度是8字节，隐式的函数声明会默认截断成32位，4字节，导致丢失字节，返回了非法的内存地址，引发coredump。

编译参数要添加选项-Wmissing-prototypes，当出现未声明的函数，会发出warning warning: no previous prototype for *** [-Wmissing-prototypes]。

参考